The present invention relates in general to means for activating the compass movement of a heading sensor having a magnetic compass to overcome bearing friction and free the compass movement to ensure tracking of small changes in heading, where heading changes are very slow and may cause the compass to stick, giving incorrect readings. More particularly, the present invention relates to compass dither means for activating a magnetic compass in the heading sensor of a marine seismic streamer or like application wherein the heading sensor changes heading very slow, to agitate the compass by momentarily vibrating it and thereby reduce errors arising from bearing friction and the like.
Heretofore, magnetic compasses have been employed in marine seismic streamers incorporated in an underwater heading sensor, for example, having am magnetic compass in a small cylindrical housing, which is pressure compensated and is electronically interrogated periodically to provide outputs signals indicating the compass heading of the seismic streamer or underwater owed array. In such seismic streamer and underwater towed array applications, it will be appreciated that frequently the changes in heading are very slow and it has been recognized that bearing friction inherent in the magnetic compass may cause the compass to stick so that very slow heading changes would not produce proper change in the compass heading and thus may give incorrect readings. A heading sensor having a magnetic compass incorporated therein, which is typical of those presently used in seismic streamers and underwater towed arrays, which is completely enclosed and provided with electronic circuitry so that readings are taken electronically at periodic intervals, may be of the type commercially produced by DigiCourse, Inc. of New Orleans, LA under its trademark DIGICOURSE and designed as a miniature underwater heading sensor, which is a rugged magnetic compass in a miniature cylindrical housing designed primarily for such seismic streamers and underwater towed array applications, constructed to withstand high shock and vibration levels that may occur during shipboard handling and implacement and retraction of the cable and array assembly. In such sensors, the magnetic compass is optically scanned and electrically interrogated, for example, once each 12 seconds, to determine the present heading of the sensor by a low current transmission line which may extend several hundred meters in length through the cable and lead portions of the seismic streamer assembly and provide pulse trains indicating the heading in degrees relative to magnetic north. Some examples of such headings sensor are disclosed in U.S. patents granted to DigiCourse Inc or the Laitram Corporation, including U.S. Pat. Nos. 3,746,842, 4,471,534, 4,093,946, 3,888,016, 3,833,901, 4,117,602 and 4,414,754.
The present invention involves means to electronically agitate the compass of a seismic streamer or underwater towed array heading sensor such as the type of heading sensor just described, by including a coil of wire surrounding the center of the heading sensor and the electronics to supply pulses of current to the coil to periodically generate a magnetic field which rotates the compass movement and effects such agitation as will vibrate the compass causing it to move slightly to overcome the effects of static friction. In the type of underwater heading sensor compass unit wherein the readings are taken electrically at periodical intervals, for example by interrogating the compass one each 12 seconds, the compass agitation system of the present invention involves pulsing the coil surrounding the compass housing during the period between interrogations to determine the compass readings, and thus electromagnetically agitate a compass card to avoid static friction affects and improvement accuracy of readings. In a preferred example, to be sure of some movement of the compass card, the agitated coil is pulsed in both directions during an agitating time period occurring between successive heading reading interrogations.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed declaration, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.